Secretory IgA is the predominant immunoglobulin in mucosal secretions and forms the first specific immunologic defense against infection of mucosal tissues. Produced by subepithelial plasma cells, this dimeric immunoglobulin is specifically taken up and transported across the epithelium via the polymeric immunoglobulin receptor (secretory component). The goal of the proposed research is to understand, at the molecular level, the mechanisms by which this transport is achieved, how it is regulated by the cell, and how this process is coordinated with other elements of the mucosal immune system. To evaluate the role of receptor structure in this process, we will construct a series of receptor mutants by site directed mutagenesis. The ability of these mutants to interact with ligand and to undergo transepithelial transport will be tested by expression of mutant receptors in Madin-Darby Canine Kidney (MDCK) epithelial cells. We have recently demonstrated that phosphorylation of the receptor by an unidentified serine kinase is essential for its translocation from the basolateral to the apical surface of the epithelium, suggesting that this process is subject to cellular regulation. We will begin to characterize this kinase at the cellular level, using a panel of inhibitors that are specific for each of the known serine/threonine kinase families. Finally, we will examine the role of cytokines, particularly interferon gamma, tumor necrosis factor alpha and interleukin 4 in IgA transport, with particular attention to their effects on pIgR kinase activity. This will be accomplished using the human epithelial cell line Caco-2, cultured on permeable filter supports. These studies will provide insight into the mechanisms of mucosal host defense, and allow us to better understand the pathogenesis of mucosal diseases such as Crohn's disease, ulcerative colitis and coeliac disease.